Superconducting digital mixer

ABSTRACT

Digital mixers which permit mixing of asynchronous signals are constructed of Rapid Single Flux Quantum (RSFQ) logic elements. The logic elements may include an RSFQ non-destructive readout cell (NDRO), an RSFQ D flip-flop, an RSFQ XOR circuit, and an RSFQ T flip-flop. A binary tree arrangement of T flip-flops can be used to provide in-phase and quadrature phase-divided replicas of a reference signal. The mixing elements can be either an XOR circuit, a dual port NDRO circuit functioning as a multiplexer or an RS type NDRO functioning as an AND gate. The RSFQ logic elements utilize Josephson junctions which operate in superconducting temperature domains.

GOVERNMENT CONTRACT

This invention was made with Government support under Contract Numbers N00014-02-C-0005, N00014-03-C-0082 and N00014-03-C-0370 awarded by the Department of the Navy. The Government has certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention described herein relates to the field of superconductivity, and more specifically relates to circuits and techniques for implementing digital quadrature mixers using Josephson junctions.

2. Related Art

Josephson junctions are quantum-mechanical circuit elements of superconducting devices. The Josephson effect in particular results from two superconductors acting to preserve long-range order across a barrier, such as an insulating barrier. With a thin enough barrier, the phase of the electron wave function in one superconductor maintains a fixed relationship with the phase of the wave function in another superconductor. This linking up of phases is called phase coherence.

A Josephson junction is the interface between two superconducting materials separated by a non-superconducting barrier. A current may flow freely within the superconductors but the barrier prevents the current from flowing freely between them. However, a supercurrent may tunnel through the barrier depending on the quantum phase of the superconductors. The amount of supercurrent that may tunnel through the barriers is restricted by the size and substance of the barrier. The maximum value the supercurrent may obtain is called a critical current of the Josephson junction.

Josephson junctions have two basic electrical properties. The first is that the junctions have inductive reactance. That is, similar to inductors, the voltage difference across the junction is related to the time rate of change of the current. The second is that a constant voltage across the junction will produce an oscillating current through the barrier, and vice versa. Thus, Josephson junctions convert a direct current voltage to an alternating current.

A family of logic/memory devices were proposed using Josephson junctions the IEEE Transactions on Applied Superconductivity, Volume 1, Number 1, March 1991, by K. K. Likharev and V. K. Semenov in an article entitled, RSFQ Logic/Memory Family: A New Josephson junction Technology For Sub-Terahertz-Clock-Frequency Digital Systems. That article is hereby incorporated by reference in its entirety into specification of this application.

RSFQ circuits are widely recognized as the fastest digital circuits in any electronic technology, and this is also true of RSFQ digital mixers. The digital mixers described in the present invention constitute the first practical circuits for the implementation of digital mixers in a complete RSFQ digital receiver system, and have been demonstrated for clock speeds up to 40 GHz.

Prior art attempts at producing digital mixers in the superconducting domain required synchronism between an incoming signal and a reference signal.

BRIEF SUMMARY OF THE INVENTION

The invention described herein is related to circuits and techniques for implementing digital mixers utilizing Josephson junction technology, which don't require synchronism between the incoming signal and a reference signal.

The purpose of the invention is to provide a digital quadrature mixer, which overcomes the problems of the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exemplary embodiment of an XOR based digital Mixer in accordance with one aspect of the invention.

FIG. 2 is an exemplary embodiment of a streaming Mixer in accordance with one aspect of the invention.

FIG. 3A is a diagram of a dual port NDRO cell, which can be utilized for the multiplexer of FIG. 2.

FIG. 3B is a block diagram of the circuit of FIG. 3A.

FIG. 4 is a diagram of a digital Mixer utilizing an NDRO cell to perform AND functionality asynchronously.

FIG. 5 is a circuit diagram of a resettable T flip-flop used in connection with the circuits of FIGS. 1, 2 and 4.

FIG. 6A is a diagram of a Reset-Set (RS) type NDRO cell used in the circuit of FIG. 1.

FIG. 6B is a partial Moore diagram of an RS-type NDRO cell used in FIG. 6A.

FIG. 7A is a circuit diagram of an XOR-type streaming mixer.

FIG. 7B is a diagram of a NDRO Multiplexer/Demultiplexer cell used in the XOR-type streaming mixer of FIG. 7A.

FIG. 8A is a circuit diagram of a D flip-flop used in the circuits of FIGS. 1, 2 and 4.

FIG. 8B is a partial Moore diagram of the circuit shown in FIG. 8A.

FIG. 9 is an exemplary layout of a digital I/Q Mixer illustrated in FIG. 1.

FIG. 10 is an exemplary layout of a digital I/Q Mixer utilized in FIG. 2.

FIG. 11 is an exemplary layout of a digital I/Q Mixer shown in FIG. 4.

FIG. 12 is a block diagram of a correlation receiver in accordance with one aspect of the invention.

FIG. 13 shows portions of the circuit of FIG. 12 in more detail.

DETAILED DESCRIPTION OF THE INVENTION

An important component of any digital receiver is a digital I/Q Mixer for converting narrowband (˜5 MHz) signals down from a few GHz. To achieve this goal with maximum efficiency the invention uses a circuit that is similar in principle to the Gilbert quadrature mixer. See article by B. Gilbert, “A Precise Four Quadrant Multiplier With Sub nanosecond Response,” IEEE. J. Solid-State Circuits, Vol. SC-3, pp. 365-373, December 1968. The basic idea of this mixer is to use square waves as a local oscillator signal instead of sine waves. The mathematical representation of a square wave is G(t)=sign[sin(ω_(LO)·t)], where ω_(LO) is a local oscillator frequency. The digital version of such a mixer is comparably easy to implement in RSFQ in case of single-bit coding, such as at the output of a delta-sigma modulator.

The first implementation of a square wave digital mixer in RSFQ is shown in FIG. 1. The right side of the device on the block diagram serves as a single-bit square wave generator with quadrature outputs. Although, FIG. 1 is shown as a two channel I/Q Mixer, the principles of the invention apply to a single channel mixer as well. As shown, the binary tree of resettable T flip-flops creates two (I and Q) local oscillator signals with 90.degree. relative phase shift. The T flip-flops control RS-type NDRO cells, which, in turn, create digital square waves turning on and off a stream of SFQ pulses. A modulated signal gets mixed with 90° shifted square waves in XOR cells, producing I channel and Q channel output products. An RS-type NDRO cell and a T flip-flop is used, instead of a T-type NDRO cell, in order to avoid a possible collision between the NDRO read-out pulse and reference pulse. Such a collision may cause a wrong phase shift between I and Q local oscillator signals. If such a problem occurs, the only way to correct it is by applying a RESET signal, and one would not want to do that too often. Despite its simple design, this version of the mixer has issues with timing, limiting its performance.

To avoid this problem, we have designed a second novel mixer performing single-bit-stream XOR multiplication (FIG. 2). In this case, we use the simple fact that A XOR 0=A and A XOR 1=Ā. A Rapid Single-Flux-Quantum (RSFQ) D flip-flop with complementary outputs (DFFC) (schematics and optimal parameters are in Kirichenko et al., “A 4-bit Single Flux Quantum Decoder,” IEEE Transactions on Applied Superconductivity, Vol. 5, No. 2, p. 2857, June 1995), converts the modulated signal into a single-bit data stream along with its inverted (complementary) representation. Multiplexing direct and inverted data outputs to the proper channel, it performs digital Quadrature signal down-conversion. After the modulated signal passes through a D flip-flop with complementary outputs (DFFC), it becomes asynchronous. Multiplexing direct and inverted data outputs to the proper channel, we effectively perform digital I/Q signal down-conversion. The multiplexing is done by two multiplexer cells controlled by the same T flip-flop binary tree as in FIG. 1, providing a 90° phase shift between I and Q channels. Again this circuit can be applied to single channel mixing as well as to I/Q quadrature mixing.

The multiplexer cell is shown in FIG. 3. This cell basically comprises a dual-port NDRO cell. From this cell, a designer can build either a demultiplexer by merging inputs A and B, or a multiplexer by merging outputs A and B. See article by Kirichenko et al., “A 4-bit Single Flux Quantum decoder,” IEEE Transactions on Applied Superconductivity, Vol. 5, No. 2, p. 2857, June 1995.

We have designed and fabricated the FIG. 3 version of the digital I/Q Mixer using the standard HYPRES 1 kA/cm² fabrication process. The same design was also converted to the standard HYPRES 4.5 kA/cm² fabrication process.

The multiplexing is done by two 2.times.1 RSFQ switches. The basic switch cell shown in FIG. 3A comprises a dual-port RSFQ Non-Destructive Read-Out (NDRO) cell with merged outputs, i.e., electrically connected outputs 1, 2. Applying an SFQ pulse to the input “Set A” causes the switch to connect input terminal “In A” to the output terminal “Out A” and disconnect terminal “In B”. Applying an SFQ pulse to the input “Set B” causes the switch to connect input terminal “In B” to the output terminal “Out B” and disconnect terminal “In A”. The optimized parameters for the cell in FIG. 3A are in Table 1.

TABLE 1 Optimized parameters for the dual-port NDRO cell, normalized to reference values in the top row. Critical current 0.125 mA Inductance 2.54 pH Bias current 0.125 mA J1 2.94 L1 1.56 I1 1.92 J2 2.78 LQ1 0.60 J3 2.16 LQ2 1.22 J4 1.95 LJ1 0.03 J5 1.25 LJ4 0.04 J6 1.60 LJ5 0.06 J7 1.88 LJ6 0.29 J8 1.55 LJ7 0.50 J9 2.07 LJ8 0.01 J10 2.89 LJ9 0.51 LJ10 0.34

Both switches are controlled by a resettable T flip-flip binary tree. See description of T1 cell in S. Polonsky, et al., “Single Flux Quantum T flip-flop and its possible applications”, IEEE trans. On Appl. Supercond., vol. 4, p. 9, 1994, for the schematics and optimal parameters of the resettable TFF. The TFF tree converts a periodic reference signal into a control sequence of the switches, effectively creating two 90-degree phase-shifted Local Oscillator square-wave signals of a half reference signal frequency.

FIG. 4 is a diagram of a digital I/Q Mixer utilizing an NDRO cell to perform an AND function asynchronously. Like the other mixer designs, a reference clock drives a binary tree of T flip-flops with the outputs of the T flip-flop driving respective NDRO cells which perform an AND function with the data arriving from the delta-sigma modulator over a type D flip-flop.

Each of the three mixers described heretofore have their advantages and drawbacks. The circuit shown in FIG. 4, employing an AND operation has a poor signal-to-noise ratio because of a DC component in the local oscillator. However, it is feasible for a multi-bit implementation.

The digital I/Q Mixer employing XOR operation shown in FIG. 1, has good gain and a good signal-to-noise ratio but there is no obvious multi-bit implementation.

The streaming I/Q Mixer shown in FIG. 2, takes care of timing and synchronization issues, but may be hard to design for multi-bit data streams. Thus, each of the three mixers described is preferred for a particular application.

The XOR mixer shown in FIG. 1, utilizes a T flip-flop, an RS-type NDRO, an XOR cell and a D flip-flop (single output).

The streaming I/Q Mixer shown in FIG. 2 utilizes the multiplexer shown in FIG. 3 and a D flip-flop with complementary outputs.

The AND gate mixer shown in FIG. 4, utilizes T flip-flops, a multiplexer, such as that shown in FIG. 3, and a D flip-flop with complementary outputs. Each of these elements utilized to construct the mixers described so far will now be described in more detail.

FIG. 5 shows a circuit for construction of an SFQ resettable toggle flip-flop as used in the construction of the circuit of FIG. 1. The parameters for this schematic are found in the Polonsky article, S. Polonsky et al., “Single Flux, Quantum B Flip-flop and Its Possible Applications”, IEEE, vol. 4, no. 1, March 1994, p. 9.

The normalized “Personal Superconducting Circuit ANalayzer” (Polonsky, S.; Shevchenko, P.; Kirichenko, A.; Zinoviev, D.; Rylyakoy, A., “PSCAN'96: New Software for Simulation and Optimization of Complex RSFQ Circuits”, IEEE Transactions on Applied Superconductivity, Volume 7, Issue 2, June 1997 Page(s): 2685-2689) (PSCAN) units are normalized to 125 pA for junction critical currents in and bias current values I, and to 2.63 pA for inductance values L.

FIG. 6A is a diagram of an RS-type NDRO cell used in FIG. 1.

This cell functions as a Non-Destructive Read-Out with a single bit memory. One can change the state of the NDRO by applying the Reset or Set inputs. If the cell is in state “1”, then the Read input pulse goes to the Output. If the cell is in state “0”, then the Read input pulse is prevented from going to the Output.

The normalized PSCAN values for the circuit of FIG. 6A are as follows: J1=3.15, J2=2.19, J3=2.21, J4=2.63, J5=1.32, J6=2.34, J7=2.39, J8=2.54, J9=2.02, J10=2.13, I1=1.80, I2=2.42, I3=1.93, LQ1=0.16, LQ2=0.16, L1=1.37, L2=0.48, L3=0.10, L4=0.80, L5=0.53, L6=1.68, L7=0.66, L8=0.61, L9=0.32, LJ2=0.80, LJ3=0.49, LJ4=0.21, LJ5=0.16, LJ6=0.25, LJ7=0.19, XN=1.00.

FIG. 7A is a circuit diagram of an XOR-type streaming mixer, indicated by the MUX cell in FIG. 2.

FIG. 7B is a diagram of a Multiplexer/Demultiplexer cell used in the XOR circuit of FIG. 7A.

This RSFQ logic circuit functions as a multiplexer or demultiplexer, combining two input pulse streams into a single output stream or conversely. This was described in the U.S. Pat. No. 5,982,219, invented by A. Kirichenko (1999).

The normalized PSCAN values for the circuit of FIG. 7B are as follows: L1=4.51, L2=1.77, L3=0.37, L4=0.60, L5=0.30, L6=0.62, LC1=1.14, LC2=0.75.

FIG. 8A shows a circuit for an SFQ D flip-flop as used in the construction of the circuit of FIGS. 1 and 4 in which the complementary output is not used, and in FIG. 2 which uses the complementary output. If the data input is “1” (i.e., an SFQ pulse) then the True output gives “1” and the Complement gives “0”. If the data input is “0” (i.e., no SFQ pulse), then the True output gives “0” and the Complement gives “1”.

The normalized PSCAN values for the circuit of FIG. 8A are as follows: I1=2.04, I2=4.45, I3=0.83, J2=1.41, J3=1.41, J4=1.96, J5=2.42, J6=2.94, J7=2.82, J8=2.43, J9=1.96, J10=1.00, L1=1.50, L2=0.30, L3=0.31, L4=0.74, L5=0.70, L6=2.28, L7=1.20, L8=1.20, L9=0.94, L10=2.00, L13=1.00, LJ2=0.08, LJ3=0.29, LJ4=0.19, LJ5=0.09, LJ7=0.45, LQ=0.23, LQ2=0.02.

FIG. 8B shows a partial Moore diagram of the D flip-flop used in FIG. 1.

FIG. 9 is an exemplary layout of a digital I/Q Mixer of the type shown in FIG. 1.

FIG. 10 is an exemplary layout of a digital I/Q Mixer of the type shown in FIG. 2.

FIG. 11 is an exemplary layout of a digital I/Q Mixer of the type shown in FIG. 4.

FIG. 12 is a block diagram of a correlation receiver in accordance with one aspect of the invention.

Turning to FIG. 13, Raw analog RF input is applied to an analog to digital convener (ADC) preferably to an oversampled delta modulator or delta sigma modulator. The output of the ADC is passed to a D flip-flop with complementary outputs and then to a multiplexer MUX. The output of the multiplexer is then passed to a chain of T flip-flops TFF1-TFF13 in this example and the output (Digital Output) is taken from the last T flip-flop. The Digital Output is passed to another D flip-flop with complementary outputs to control the phase shift imported to a frequency reference by a string of alternating inverters and T flip-flop (TFF 1-4) cells. The output of the last T flip-flop connects to the set/reset inputs of the multiplexer of the streaming mixer.

While various embodiments of the present invention have been illustrated herein in detail, it should be apparent that modifications and adaptations to those embodiments may occur to those skilled in the art without departing from the scope of the present invention as set forth in the following claims. 

1. A digital mixer comprising: a. a first input adapted to receive an incoming digital signal representing an oversampled representation of a corresponding analog signal; b. a second input adapted to receive a digital reference signal; and c. at least one unclocked data non-destructive readout rapid single flux quantum circuit having dual sets of input ports configured for mixing said incoming digital signal and said digital reference signal, each of said incoming digital signal and said digital reference signal being presented at respective one of said dual sets of input ports, and having a single merged output port adapted for providing an output spectrum having mixed signal products.
 2. The mixer of claim 1 in which said incoming digital signal is a complementary signal generated by a circuit comprising a D flip-flop with complementary outputs.
 3. The mixer of claim 2 in which said D flip-flop is a Rapid Single Flux Quantum circuit.
 4. The mixer of claim 1, wherein said mixer comprises two unclocked data non-destructive readout rapid single flux quantum circuits, and a binary tree of T flip flops adapted to generate respective in-phase and quadrature reference signals, said two unclocked data non-destructive readout rapid single flux quantum circuits respectively receiving different ones of said in-phase reference signal and said quadrature reference signal.
 5. A digital receiver using the mixer of claim
 1. 6. The mixer of claim 1, further comprising at least one counter configured to receive a merged output of said circuit.
 7. A mixer for mixing a digital reference signal with an incoming digital signal comprising: a. an input adapted for receiving a plurality of digital reference signals which are respectively phase shifted; b. at least two circuits for mixing, each receiving a respective one of the plurality of digital reference signals which are respectively phase shifted and its respective complement, and a representation of the incoming digital signal and its complement, each of said circuits for mixing comprising an unclocked, dual data port, merged output, non-destructive readout, rapid single flux quantum circuit; and c. an output from each of said circuits for mixing for providing an output spectrum having the mixed signal products representing a digital product of the respective one of the plurality of the digital reference signals which are respectively phase shifted and a representation of said incoming digital signal.
 8. The mixer of claim 7 in which at least one of said at least two mixing circuits is a Rapid Single Flux Quantum circuit.
 9. The mixer of claim 7 in which said at least two circuits for mixing have an XOR gate response.
 10. The mixer of claim 7 in which each of said unclocked, data dual port merged output, non-destructive readout, rapid single flux quantum circuits acts as a multiplexer.
 11. The mixer of claim 7 in which said incoming digital signal is processed by a circuit comprising a D flip-flop.
 12. The mixer of claim 11 in which said D flip-flop is a Rapid Single Flux Quantum circuit.
 13. The mixer of claim 7 in which said circuit for receiving said digital reference signal comprises a binary tree of T flip flops.
 14. The mixer of claim 13 in which said T flip-flops are Rapid Single Flux Quantum circuits.
 15. A digital receiver using the mixer of claim
 7. 16. A digital RF receiver using the mixer of claim 7, in which the incoming digital signal is the output of a single bit oversampled analog to digital converter having an input which receives an RF input signal at the input of the digital converter.
 17. A method mixing a digital reference signal with an incoming digital signal comprising the steps of: a. providing the incoming digital signal; b. mixing the digital reference signal with the incoming digital signal without synchronizing the incoming digital signal to the digital reference signal through at least one digital mixer comprising a multiplexer having an unclocked, dual data port, merged output, non-destructive readout, rapid single flux quantum circuit, configured such that the digital reference signal multiplexes the incoming digital signal over time; and c. producing, by the at least one digital mixer, at least one digital data stream having an output spectrum containing the digital product of the incoming digital signal and the digital reference signal.
 18. The method of claim 17 in which the digital mixer comprises at least one rapid single quantum flux cell circuit which operates at superconducting temperatures.
 19. The method of claim 17 in which the digital mixer comprises one or more Josephson junctions.
 20. A streaming multiplier circuit having an XOR output response, comprising a D flip-flop with complementary outputs driving a first pair of inputs of an asynchronous multiplexer, and a second pair of inputs of the asynchronous multiplexer adapted to receive a complementary input signal, the asynchronous multiplexer comprising an unclocked data non-destructive readout rapid single flux quantum circuit having a single merged output port adapted for providing an output spectrum having mixed signal products.
 21. A digital mixer using the streaming multiplier of claim
 20. 22. A digital RF receiver using the multiplier of claim 20, in which an incoming signal to the streaming multiplier circuit is an output of a single bit oversampled analog to digital converter having an input which receives an RF input signal at the input of the analog to digital converter.
 23. A cross correlator using the multiplier circuit of claim
 20. 24. A digital receiver using the cross correlator of claim
 23. 25. A method for digitally mixing an incoming digital signal with a digital reference signal, comprising: a. asynchronously receiving an incoming digital signal representing an oversampled representation of a corresponding analog signal and a digital reference signal; b. digitally mixing the asynchronously received incoming digital signal and the digital reference signal, with an unclocked non-destructive readout rapid single flux quantum circuit having a single merged output port; and c. outputting a mixed digital signal from the single merged output port, having a spectrum representing mixed signal products of the incoming digital signal and the digital reference signal.
 26. The method according to claim 25, wherein the unclocked non-destructive readout rapid single flux quantum circuit receives the asynchronously received incoming digital signal and its complement, and the digital reference signal and its complement.
 27. The method according to claim 26, wherein the complement of the incoming digital signal is generated by a rapid single flux circuit comprising a D flip-flop with complementary outputs.
 28. The method according to claim 25, further comprising: a binary tree of T flip flops adapted to generate respective in-phase and quadrature phases of the digital reference signal; and a second unclocked non-destructive readout rapid single flux quantum circuit having a single merged output port which outputs a mixed digital signal having a spectrum representing mixed signal products of the incoming digital signal and the digital reference signal, the unclocked non-destructive readout rapid single flux quantum circuit having a single merged output port and the second unclocked non-destructive readout rapid single flux quantum circuit having a single merged output port receiving respectively different phases of the digital reference signal.
 29. The method according to claim 25, wherein the unclocked non-destructive readout rapid single flux quantum circuit having a single merged output port has an XOR gate response.
 30. The method according to claim 25, wherein the unclocked non-destructive readout rapid single flux quantum circuit having a single merged output port comprises Josephson junctions, further comprising cooling the Josephson junctions to a temperature at which they are superconductive. 